1. Field of the Invention
The present invention relates to a method of growing an epitaxial film, a wafer supporting structure, and a susceptor. More specifically, the present invention relates to a method of growing an epitaxial film in which an epitaxial film is grown on a front surface of a semiconductor wafer by vapor-phase epitaxial growth, a wafer supporting structure, and a susceptor.
2. Description of Related Art
With increasing wafer diameter these days, a single-wafer type vapor-phase epitaxial growth apparatus is widely used, in order to grow an epitaxial film on a front surface of a silicon wafer. In a single-wafer type apparatus, a silicon wafer is first placed on a susceptor installed in a passageway-shaped reactor (chamber). Subsequently, when being heated by a heater provided external to the reactor, the silicon wafer is reacted with a variety of source gases (raw material gas and reactive gas), which pass through the reactor. Thereby, an epitaxial film is grown on a wafer front surface.
A widely used susceptor has a circular shape from a plan view, on which a single wafer is mountable. The type of susceptor is used in order to evenly heat a wafer having a large diameter, such as, for example, a circular silicon wafer having a diameter of 300 mm, and to supply source gas on an entire wafer front surface; and thereby to evenly grow an epitaxial film. A wafer housing portion having a recess shape is provided in a central portion of an upper surface of the susceptor, so as to house a silicon wafer having front and rear surfaces positioned horizontally. A recent susceptor generally supports a silicon wafer in a boundary area with a chamfered surface of a rear surface of the silicon wafer (for example, Related Art 1). In order to provide a wafer supporting position in the boundary area, one method is to evenly reduce a thickness of a central portion of a bottom plate of the wafer housing portion, and thereby to provide a step around an external peripheral portion of the bottom plate. The other method is to cut out in a circular shape the central portion of the bottom plate of the wafer housing portion, and thereby to provide the bottom plate having a ring shape. The boundary area means an area of less than 1 mm internally and externally in a direction of a wafer diameter, centering a boundary line with the chamfered surface of the rear surface of the silicon wafer.
Silicon carbide (SiC) has conventionally been employed as material of a susceptor front surface. Thus, the susceptor has a greater hardness than the silicon wafer. Further, the susceptor has a higher coefficient of thermal expansion than the silicon wafer, as the coefficient of thermal expansion of SiC is 4.8×10−6/k and that of Si is 2.5×10−6/k. Due to the difference in the coefficient of thermal expansion, the boundary area of the wafer rear surface and an upper edge of an internal periphery of the external peripheral portion of the bottom plate of the wafer housing portion are in friction at a time of epitaxial growth, when a temperature inside the chamber is high. As a result, scratches are caused in the boundary area of the silicon wafer, which is softer than the susceptor. The scratch has a groove-like shape similar to a hangnail caused in a portion that rims a base of a nail (a rip or a hangnail injury). A planar shape of the scratch is a line, a dot, and the like. A cross-sectional shape thereof is a V-shaped notch and the like.    Related Art 1: Japanese Patent Laid-open Publication No. 2003-229370
With microprocessing in a device process, however, problems described below occur when scratches exist on the rear surface of the silicon wafer in the boundary area between the chamfered surface and the wafer rear surface. Specifically, when the silicon wafer is immersed in an etching solution or ultrapure water in the device process, particles are generated from the scratched portion. The particles then move to a front surface side of the wafer, and are deposited on the wafer front surface (device formed surface). Thus, a yield in the device process is reduced.
Recently, device manufacturers demand ultra-high flatness for a front surface of an epitaxial film. To meet the demand, it is necessary to prevent a film from forming on a rear surface in an external periphery end portion (up to around 6 mm from an edge) of the wafer rear surface, as source gas moves to the wafer rear surface side at the time of epitaxial growth. The film forming phenomena on the rear surface fluctuates a distribution of film forming on the rear surface in the wafer external peripheral area, and fluctuates a total thickness of the wafer; thus preventing the ultra flatness of the epitaxial wafer. To prevent the film forming phenomena on the rear surface is thus another problem.